Method of using a catalytic filter

ABSTRACT

A method of using an improved catalytic filter material for removing contaminants such as NO x  from a fluid stream. The filter employs composite fibers of expanded polytetrafluoroethylene (PTFE) filled with catalytic particles. The composite fibers are chopped into staple fibers and made into a felt material. Preferably, the felt material is then laminated on at least one side with a protective microporous membrane. The combined filter removes both macro-particles, such as dust, from the filter stream before the dust can clog active catalytic sites and effectively catalytically coverts undesirable contaminants in the fluid stream to acceptable end products.

RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 08/515,195 filed Aug. 15, 1995 now U.S. Pat. No. 5,620,669.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a chemically and/or catalyticallyactive particulate and gas filtration materials, such as those used influe gas cleaning processes.

2. Description of Related Art

Catalytic filters are employed for a variety of gas filteringapplications. Typically these filters combine some form of catalyticmaterial (e.g., TiO₂, V₂ O₅, WO₃, Al₂ O₃, MnO₂, zeolites, and transitionmetals and their oxides) within some matrix. As the gas passes over orthrough the matrix, contaminants within the gas will react with activesites on the catalyst to convert the contaminants to a more desirableby-product. Examples of such include:

    ______________________________________                                        Contaminant Catalyst      Resulting Product(s)                                ______________________________________                                        NO.sub.x, NH.sub.3                                                                        TiO.sub.2, V.sub.2 O.sub.3, WO.sub.3                                                        N.sub.2 + H.sub.2 O                                 CO          Al.sub.2 O.sub.3, Pt                                                                        CO.sub.2                                            Dioxin/Furan                                                                              TiO.sub.2, V.sub.2 O.sub.3, WO.sub.3                                                        CO.sub.2, HCl                                       O.sub.3     MnO.sub.2     O.sub.2                                             ______________________________________                                    

Examples of various previous attempts to produce a catalytic filterdevice include those set forth in U.S. Pat. No. 4,220,633 to Pirsh; U.S.Pat. No. 4,309,386 to Pirsh; JP 4-156479 to Norio Maki; EP 0,470,659 toEkkehard, Weber; U.S. Pat. No. 4,053,557 to Kageyama Yoichi; U.S. Pat.No. 5,051,391 to Tomisawa et al.; U.S. Pat. No. 4,732,879 to Kalinowskiet al.; DE 3,633,214 A1 to Dr. Hans Ranly.

In certain cases (e.g., U.S. Pat. No. 4,220,633 and U.S. Pat. No.4,309,386) the filters have to collect substantial amounts of dust, suchas that generated in a combustion process. After short collection timesof between 1 minute and 6 hours, a layer of collected dust on the dirtyside of the filter material increases the pressure drop across thefilter and the filter has to be cleaned. (In many cases in situ.) Duringthis cleaning cycle (e.g., a high energy air impulse system, a shakersystem, a reverse air system, etc.), the outer dust layer falls off anda new filtration cycle can begin. Most catalytic filter materials todayof which none are commercially available constitute a mesh of a regularwoven or non-woven filter material in which the catalytically activeparticles are inserted as a foreign body. During the cleaning cycle, inwhich the filter material is exposed to high energy input and flexing,these particles are believed to abrade the host fibers at the fiberinterception points and degrade the life of the filter.

Furthermore, inserted catalytic particles have the disadvantage thatthey increase the pressure drop of the filter material. In a filterwhich is used for particle collection, an optimal percentage of thefilter will be occupied by fibers. If less fibers are used, the filterbecomes weak and particulate collection will decrease. If more fibersare used, the filter will become stronger and collect dust particles ata higher efficiency but the pressure drop across the filter willincrease above tolerated levels. Since catalytic particles on thesurface of fibers will not increase fiber strength but rather fiberdiameter, one will have to use at least the same amount of fibers asused for the original filter to obtain sufficient strength. In thiscase, the pressure drop increases significantly. On the other hand, ifless fibers are used to keep pressure drop consistent, the resultingfilter will be weaker. In general it can be said that the more volume ina filter is occupied by noncatalytic constituents, the lower thecatalytic activity per unit volume of the filler or the higher thepressure drop across the filter. In addition, the contact of thepollutant dust with the catalyst particles in the filter medium willdecrease catalyst activity due to catalyst pore or active site clogging.In cases where glue is employed to help anchor the catalyst in placewithin the material, the glue tends to clog active sites on the catalystand diminish its effectiveness. In cases in which catalyst particles areglued to solid surfaces, gases can only access the catalyst from theside which is directly facing to the fluid stream.

A number of other instances (e.g., Japanese Patent Application JP4-235718 to Vilene Co., Ltd.) employ integrated catalysts and supportmatrix. While abrasion from loose particles can be reduced or avoidedwith this approach, these devices continue to have significant problems.First, many of these materials are relatively weak and tend to provideinadequate catalyst retention and/or are easily damaged during handlingand use. This condition usually worsens as the quantity of catalyst isincreased in the matrix.

Second, the catalytic filter must be thin and open enough to assure thatgas can readily reach the active catalytic sites. Unfortunately,providing a thin and open structure decreases the strength and integrityof the filter material even further. While reinforcing materials orthicker or denser materials might be employed in the filter to increaseits strength, the filter will undergo a resulting decrease in gasremoval efficiency since there will be fewer fully exposed activecatalytic sites for gas contact. Furthermore, denser or thicker materialwill cause an undesirable increase in the pressure drop through thematerial. These problems are particularly evident in Japanese PatentApplication JP 4-235718 to Vilene, where it is taught that the catalyticmaterial may need to have holes punched into it in order to produceadequate flow-through properties. Of course the use of through-holes isnot entirely acceptable since gas flowing directly through macroscopicholes in the material will not contact any catalyst.

Third, contamination is a serious problem with virtually every previouscatalytic filter device. Although by definition a catalyst is notconsumed during the catalytic reaction, until the present inventioncatalytic filters may have limited operating lives due to particlecontamination in a fluid stream (e.g., fine dust particles, metals,silica, salts, metal oxides, hydrocarbons, water, acid gases,phosphorous, alkaline metals, arsenic, alkali oxides, etc.). Overtime,these aerosols tend to become embedded within the filter matrix, thusblocking the pores of the catalyst and, therefore, minimize the surfacearea and access to the active sites of the catalyst. Unless theseparticles can be shed from the filter, the filter will rapidly diminishin efficiency until it must be replaced. As has been noted, a variety ofcleaning apparatus exists to remove dust from filter apparatus (e.g.,shaker filter bags, back-pulse filter bags and cartridges, reverse airfilter bags, etc.), but these devices are not expected to beparticularly effective at removing dust from current integratedcatalytic filter materials. This is due to the filter's overallweakness, preventing its rigorous handling; and the intricacies of thefilter structure, making it very difficult to remove particles from thematrix once they have become embedded therein.

Accordingly, it is a primary purpose of the present invention to providea catalytic filter material that is effective at catalyticallyconverting contaminants in a fluid stream. Fluid streams in thisinvention are gas and liquid streams.

It is a further purpose of the present invention to provide a catalyticfilter material that has improved strength and a more open structureover existing catalytic filter designs. Within the new filter structure,pollutant molecules can access the catalyst particles from all sides.

It is yet another purpose of the present invention to provide acatalytic filter that can be effectively cleaned, with minimumcontamination of the catalytic particles, so that the filter has anextended effective operating life.

These and other purposes of the present invention will become evidentfrom review of the following specification.

SUMMARY OF THE INVENTION

The present invention is an improved catalytic filter device for use inconverting contaminants found in a fluid stream from an undesirablesubstance, such as NO_(x), to an acceptable end-product, such as wateror N₂. The present invention differs from previous catalytic filterproducts in a number of important respects. First, the filter comprisescatalytic particles that are attached within the polymeric node andfibril structure of fibers of expanded polytetrafluoroethylene (ePTFE).Preferably, the fibers are "towed" (i.e., partially ripped apart) usinga towing process to produce an open, intertangled web of fibers. Anon-woven structure is then formed from the web of fibers. This hasproven to be a very strong and very open catalytic filter material. As aresult, the catalytic material of the present invention can be made intoa thicker filter product (i.e., 1-5 mm or more in thickness) thanprevious catalytic filters without producing an unacceptable pressuredrop. Further, the use of small particles with high surface area whichcan be accessed from all sides by polluting molecules allows for greatercatalytic activity than has been previously possible.

Another important improvement of the present invention is that itsincreased strength and its intimate adhesion to catalytic particles makethe material perfect for use in demanding environments, such as withshaker bag, reverse air, or pulse-jet filter cleaning assemblies. Sincetowed ePTFE fiber material is quite strong and resistant to abrasion, itcan readily withstand the flexing and rigorous handling of self-cleaningfilter apparatus. Moreover, the adhesion of the catalytic particles tothe node and fibril structure of the ePTFE greatly reduces the abrasionthat might otherwise occur by the rubbing of particles against fabricduring the operation and cleaning of the filter apparatus.

To further improve the operative life of the present invention, it ispreferred that a microporous membrane of expanded PTFE be mounted on atleast the upstream side of the filter apparatus when particles arepresent in the gas stream (when no particles are in the gas stream, themembrane may not be necessary). The ePTFE membrane provides a pre-filterto separate dust particles and other contaminants from the gas stream.The result is that dust particles will form into a cake on the outsideof the ePTFE membrane and will not become embedded within the catalyticfilter material. Shaker or back-pulse cleaning becomes easy under thesecircumstances since the dust will readily separate from the microporousePTFE membrane with PTFE's low surface energy. The enhanced cleanabilityallows the filter to be repeatedly regenerated without the performanceloss that can occur when dust begins to contaminate the catalystparticles. For some uses, it may also be useful to add a microporousmembrane that is filled with catalytic particles or other material toprovide additional levels of filtration or other useful properties. Inaddition, the addition of the ePTFE (or other) membrane to the catalyticbackup material removes the backup material from its dust removingduties. In this case, many more degrees of freedom in catalytic backupmaterial construction are gained. Filters can be made more or lessdense, with a higher or lower thickness, more or less tortuous to thegas flow, or with a higher or lower strength.

The present invention can also be used in cross flow situations in whichthe rough fibrous structure ensures good fluid mixing due to enhancedturbulent fluid flow and therefore, an intense contact between the fluidstream and the catalyst particles. Also, a fibrous structure increasesfluid-catalyst contact because boundary layer effects, which are mostpronounced on smooth surfaces, will not limit contamination gas moleculetransport to the catalyst.

A number of unique process steps also distinguish the present invention.The preferred process comprises:

1) An active catalyst with very low particle sizes is combined with aPTFE resin and then expanded to produce a structure whereby the catalystparticles are almost completely exposed to the surrounding air and onlyconnected by very fine fibrils of PTFE, giving the whole structureextensive strength and excellent reactivity with the targeted gases.

The path of the fluid passing through the filter is tortuous because nostraight pores exits.

2) The catalytic structure is cut into fine catalytic fibers withoutdestroying the node and fibril structure. Preferably this is done bytowing the fibers to make them into a tangled web of interconnectedfibers;

3) The catalytic fibers may be mixed with regular ePTFE fibers or otherfibers to provide increased strength;

4) Preferably the mixture of catalytic and ePTFE fibers is then cardedand needled into a backing material (e.g., a scrim) to obtain a needlefelt.

5) As has been noted, the needle felt material then can be mounted to amicroporous sheet of ePTFE to serve to protect the felt from dustparticle contamination in the fluid stream.

This manufacturing process allows a wide range of filter parameterchanges such as catalyst loading, filter thickness, filter permeability,flow field around the catalyst particles (a unique mixture of flowthrough and flow by with excellent gas/catalyst contact), filterstrength, and catalyst protection.

DESCRIPTION OF THE DRAWINGS

The operation of the present invention should become apparent from thefollowing description when considered in conjunction with theaccompanying drawings, in which:

FIG. 1 is a scanning electron micrograph (SEM) of a number of catalystloaded expanded polytetrafluoroethylene (ePTFE) fibers employed in thepresent invention, enlarged 100 times;

FIG. 2 is a scanning electron micrograph (SEM) of a catalyst loadedfiber employed in the present invention, enlarged 1,000 times;

FIG. 3 is a scanning electron micrograph (SEM) of a catalytic fiberemployed in the present invention, enlarged 5,000 times;

FIG. 4 is a microscopic photograph, enlarged 5:1, of a coherentcatalytic filter material of the present invention comprising a mixtureof catalyst loaded ePTFE fibers and unfilled ePTFE fibers;

FIG. 5 is a schematic representation of a tow fiber employed in thepresent invention;

FIG. 6 is a cross-section schematic representation of one embodiment ofa catalytic filter device of the present invention;

FIG. 7 is a cross-section schematic representation of another embodimentof a catalytic filter device of the present invention;

FIG. 8 is a cross-section schematic representation of still anotherembodiment of a catalytic filter device of the present invention;

FIG. 9 is a cross-section schematic representation of yet anotherembodiment of a catalytic filter device of the present invention; and

FIG. 10 is cross-sectional view of a further embodiment of a filterdevice of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The current invention is an improved catalytic filter material. Withthis filter, pollutant gas components, such as NOx, Dioxin/Furan, CO,and others, can be catalytically altered (i.e., reduced or oxidized)into non-polluting or less polluting gas components. In addition,particulates in the gas stream can be separated and collected in thefilter with high efficiencies.

The present invention is directed to a wide variety of filterapplications. The terms "filter" and "filtration" as used in the presentapplication is intended to encompass any device that blocks or trapsparticles and/or modifies particles or molecules passing through thedevice. The use of the term "fluid" in the present invention is intendedto encompass any form of readily flowing material, including liquids andgases.

FIGS. 1 through 3 show catalytic fibers 10 of the present invention atthree different magnification levels. The fiber 10 comprises an expandedpolytetrafluoroethylene (ePTFE) that is composed of polymeric nodes 12and interconnecting fibrils 14. The production of this basic expandedPTFE structure is taught in a number of patents, including U.S. Pat.Nos. 3,953,566, 3,962,153, 4,096,227, and 4,187,390, all incorporated byreference. By the terms "fibrils" and "nodes," it is intended to definea wide variety of materials such as those made in accordance with thesepatents and improvements thereto. "Fibrils" are intended to define smallpolymeric strands (such as, on the order of a few microns or less indiameter). "Nodes" are intended to define any structure attached to suchfibrils, from relatively large polymer masses or particles (such as, upto 10 to 50 microns or more across) to mere intersect points of two ormore fibrils.

Expanded PTFE has a number of desirable properties that makes itparticularly excellent filtration material. For example, expanded PTFEmaterial has many microscopic holes or "micropores" 16, such as on theorder of 0.1 to 10 μm across, that allow gases to pass through butrestricts the passage of larger materials, such as fine dust, etc. Itshould be appreciated that pore size of the fibers may be varieddramatically within the scope of the present invention ranging from lessthan 0.05 μm to over 100 μm across. Further, expanded PTFE has thedemonstrated ability to be treated to selectively allow or restrict thepassage of liquid water and/or water vapor. Finally, expanded PTFE isvery strong and inert, allowing it to be used with a wide variety ofmaterials and under a wide array of environmental conditions (includingat temperatures up to about 260° C. continuous, and even up to 290° C.for short durations).

The filter material of the present invention attaches catalyticparticles directly to the microstructure of the expanded PTFE. As can beseen in FIGS. 2 and 3, particles of catalyst 18 are attached to andwithin the nodes and fibrils of the expanded PTFE. As is explainedbelow, the catalytic particles 18 are introduced during the processingof the expanded PTFE itself so as to produce a stable microporousmaterial with the particles securely attached to the fibrils 14themselves. Preferably, the surface of the catalyst particles is coveredwith a minimal amount of PTFE so as to maximize the reactivity of thecatalyst.

The preferred material of the present invention is made in the followingmanner. A catalytic filler is incorporated into an aqueous dispersion ofdispersion-produced PTFE. The filler in small particle form isordinarily less than 40 microns in size, and preferably less than 15micron. By "particles" is meant a material having any aspect ratio andthus includes flakes, fibers and spherical and nonspherical powders.

Examples of suitable fillers for use in the present invention include:Noble (gold, silver, palladium, rhodium, etc.) or non-noble metalliccatalysts may be utilized. For example, platinum (a noble metal), ironbase ammonia decomposition catalyst, iron chromium oxide mixtures,zirconium promoted lanthanum cuprate, and the various oxides of copper,iron, vanadium, cobalt, molybdenum, manganese, and tungsten may beemployed with the instant invention. The foregoing recitation is notmeant to be exhaustive, rather, other suitable catalysts may be employedas well. Other reagent/catalyst include transition metal oxides ofnickel, zinc; alumina (particular gamma phase) silicone, zirconium,chormium, ruthenium, tin, and alkalized alumina; alkali and alkalineearth oxides and carbonates; and minerals such as dawsonite, analcite,magnesioriebeckite, feldspars, alunite, anataso, azurite, bauxite,bunsanite, gothite, hematite, iron spinel, ilmenito, malachite,manganite, manganosite, mellite, siderite, and spinel.

The filler is introduced prior to co-coagulation in an amount that willprovide 1 to 99%, and preferably 30% to 90% by volume, solid-to-solidvolume filler in the PTFE in relation to the final composite material(not including air content). The filled PTFE dispersion is thenco-coagulated, usually by rapid stirring and the coagulated filled PTFEis dried. The filled material is then lubricated with a common pasteextrusion lubricant, such as mineral spirits or glycols, and pasteextruded.

The extrudate is usually calendered, and then rapidly stretched to 1.2×to 5000×, preferably 2× to 100×, at a stretch rate of over 10% persecond at a temperature of between 35° C. and 410° C. The lubricant canbe removed from the extrudate prior to stretching, if desired.

Preferably, the catalyst-filled material of the present invention isformed into a felt. One suitable means of forming such a felt is in thefollowing manner. A catalytically active ePTFE fiber is produced from acatalytic tape of ePTFE produced from a wet mixture of 1 to 99% byvolume, preferably 30 to 90% by volume, catalyst (e.g., titanium dioxidecatalyst from BASF, Ludwigshafen, Germany) and 99 to 1% by volume,preferably 70 to 10% by volume, PTFE resin (e.g., PTFE dispersionavailable from E. I. duPont de Nemours and Co., Wilmington, Del.). Thetape is slit along its length into multiple strips, expanded, andprocessed over a rotating pinwheel to form a tow yarn. A representationof tow yarn 20 is shown in FIG. 5. As can be seen, once the yarn passesthrough the pin wheel, a "spider web" of fine fibers 22 is formed thatare connected together at random points 24 along the tow 20. This isaccomplished by puling a catalyst-filled tape of PTFE through spikedpinwheels. By providing the pinwheels with a higher velocity than thespeed of the tape, the material is slit to form the web structure shown.This process produces a very open structure with a high percentage ofopen and exposed surface area. Once the tow is formed, the tow yarn isthen chopped into short staple fibers. The staple fibers should be about0.2 to 25 cm, preferably 2 to 12 cm (and especially about 5 cm) inlength.

The staple fibers are then needle punched into a scrim backing materialto form a felt. Preferably, a woven scrim is used made from ePTFEweaving fiber (e.g., 440 decitex RASTEX® fiber, available from W. L.Gore and Associates, Inc., Elkton, Md.). The scrim preferably comprisesa thread count of approximately 16×8 threads/cm, resulting in a weightof approximately 130 g/m². It should be understood that the scrim may beproduced from catalyst-containing material was well.

The staple fibers are fed into conventional carding equipment (e.g.,available from Davis and Furber of Andover, Mass.). The carded web iscrosslapped onto a scrim and tacked together by a needle loom. The webis then crosslapped onto the other side of the scrim and needled again.The felt should be needle punched several times to interlock the staplefibers to the scrim sufficiently. This product may then be heat setwhile being restrained in the cross machine direction for severalminutes to improve the thermal stability. The final felt preferably hasa weight of approximately 300 to 3000 g/m² (with a weight of less thanabout 1500 g/m² generally preferred), an air permeability ofapproximately 4 to 60 m/min @ 11 mm water gauge, and a thickness ofapproximately 2 to 15 mm.

The felt material may then be coated with an adhesive material.Preferably the adhesive material comprises a fluorinated ethylenepropylene copolymer (FEP) aqueous dispersion (e.g., T120 available fromE. I. duPont de Nemours and Co.). Other lamination aids are PTFEdispersions, fluoropolymers, polyimides, sulfur (polyphenylene sulfide,etc.). The felt is then dried (e.g., in an oven at about 200° C.-250° C.for about 2 to 10 min).

For protection of the felt material, a layer of porous membrane may belaminated on the coated side of the felt. The preferred membranecomprises an expanded PTFE with an air permeability of about 0.3 to 200m/min @ 12 mm water gauge. Preferred air permeability is about 6 m/min @12 mm water gauge. To achieve bonding, the felt is subjected to heat andpressure to soften the dried FEP aqueous dispersion. The resultingfabric laminate had good strength between the porous expanded PTFEmembrane and the felt. Preferably the final material has an airpermeability of about 1 to 10 m/min @ 12 mm water gauge with excellentfiltration efficiency of solid particulates.

A further improvement in this material is to form the felt from acombination of both catalyst filled material and another material. Forinstance, a staple fiber may be produced from an unfilled fiber ofexpanded porous PTFE. The catalytically active ePTFE fiber and thesynthetic fiber of expanded porous PTFE may then be blended to form ahybrid mixture within a broad range of 100:1 to 1:100. Preferably themixture comprises approximately 10:1 to 1:5 catalytic to noncatalytic byweight of the two staple fibers. For most applications, a mixture of atleast about 50% catalytic fiber is ideal.

The blended hybrid mixture is then placed in a carding machine andprocessed in the manner described above. The final felt preferably has aweight of approximately 300 to 3000 g/m², an air permeability ofapproximately 4 to 60 m/min @ 12 mm water gauge, and a thickness ofapproximately 2 to 15 mm. Following lamination, the final material hasan air permeability of about 1 to 10 m/min @ 12 mm water gauge.

FIG. 4 shows the resulting structure of this hybrid material. As can beseen, the material comprises both strands of catalytic fibers 26 andstrands of unfilled fibers 28. The two kinds of strands 26, 28 arerandomly intermingled with each other. The open structure of thismaterial allows for ready air access into and around the catalyticfibers, while the non-catalytic fibers lend strength to the felt.

FIG. 6 shows how a new filter 30 of the present invention can be made.To protect the catalyst material from contamination (such as, from dustor other material blocking catalytic sites), in this embodiment, aprotective microporous membrane 32 is laminated to the catalytic filtermaterial 34. In this configuration, dust particles 36 and adsorbedpollutants on these dust particles are blocked by the protectivemembrane 32 and cannot come into contact with the catalyst particlesattached with the filter material 34. This provides significantlyimproved protection of the catalytic filter material 34 and vastlylonger effective life for the filter 30. While a variety of protectivemembranes 32 may be employed, it is particularly preferred to employ anexpanded PTFE membrane due to its exceptional filtration properties.Additionally, an expanded PTFE membrane can be readily cleaned ofaccumulated contaminants, vastly increasing the operative life of thefilter 30.

The filter illustrated in FIG. 6 is particularly suited for use intreating pollutant gases and particulates therein. For instance, byemploying a catalyst of TiO₂, V₂ O₃, and WO₃, pollutants of NO, NO₂, andNH₃ will readily be modified into H₂ O and N₂.

If the gas-catalyst reaction is instantaneous, and the catalyst is veryexpensive, it might be desirable to make only a thin catalytic layerthat may be active enough to convert all pollutent gases. In this case afurther backup or support layer can be laminated into the filterassembly. As is shown in FIG. 7, a filter 38 is shown having a catalyticfilter layer 40, a porous protective layer 42, and a sorptive layer 44.The sorptive layer 44 can be mounted either upstream or downstream ofthe protective membrane 42 or the catalytic filter layer 40. In mostcases it is preferred that the sorptive layer 44 be mounted between theprotective membrane 42 and the catalytic filter layer 40, as shown. Thesorptive layer 44 serves to absorb or adsorb other poisons andpollutants in the fluid stream. This layer may be formed from anysuitable sorptive material, including carbon filled felt or weaves, etc.

The catalytic filter of the present invention may be combined with otherfilter components with additional beneficial results. For instance, ifother gas components threaten to poison the catalytic filter layer, afurther protective catalytic layer may be inserted anywhere upstream ofthe catalytic layer, as is shown in FIG. 8. The filter unit 46 of FIG. 8employs a first catalytic layer 48 and a second catalytic layer 50.Although not always necessary, it may be desirable to include a layer ofmaterial 52 between the two layers 48, 50 to isolate the layers fromeach other and/or to provide some other function (e.g., scrim,absorption, liquid separation, further catalytic function, etc.) Again,a protective layer 54 and sorptive layer 56 are provided upstream.Although the two filter layers 48, 50 may use identical or similarcatalytic materials, it is contemplated to be particularly useful toprovide different catalytic layers so as to improve the overallfunctioning of the filter. For instance, while one catalytic layer maycatalytically reduce NO_(x), the other could catalytically oxidize CO toCO₂. In addition, a second or third layer could adsorb SO₃ which poisonssome of the other catalysts.

FIG. 9 illustrates a further example of how multiple layers offiltration material can be combined in the filter device of the presentinvention. In this embodiment, the filter device 58 comprises a firstprotective layer 60, a sorptive layer 62, a catalytic layer 64, asorptive layer 66, and a second protective layer 68. The sorptive layer66 is preferably one that can absorb or adsorb undesirable materialsfrom the fluid stream before it exits the filter device, such as acarbon-filled polymer. The use of a second protective layer 68 isbelieved to provide better containment and protection of the activelayers within the filter device 58 and to provide resistance todistortion of the filter device when it is place in a strong fluidstream or in direct contact with filter support materials such as filtercages. Ideally, the second protective layer 68 should be constructedfrom a strong, porous, and abrasion-resistant material, such as apolymer felt or mesh.

Still another example of a multiple layer construction of a filterdevice 70 of the present invention is illustrated in FIG. 10. In thisembodiment, the filter device 70 comprises a first protective layer 72,a support layer 74, and an integrated layer 76 that includes a catalyticlayer portion 78, an absorptive layer portion 80, and a protective layerportion 82. The integrated layer 76 essentially combines the protectivelayer and the active layers 78, 80 into a coherent unit. Preferably,such combination is accomplished by forming the protective layer in ascrim needled punched to form a felt of catalytic fibers. The absorptivelayer 80 may likewise be combined into the felt, such as in the form ofa needled punched filled membrane, filled fibers, loose absorptivematerial, etc. Alternatively, this integrated layer 76 may beconstructed of only the catalytic layer 78 and the protective layer 82,with the absorptive layer 80 not used or applied separately.

It should be appreciated that there are numerous permutations of filterapparatus that can be made in accordance with the present invention.Among the combinations of material contemplated by the present inventionare: (1) using catalytic felt alone; (2) covering catalytic felt withone or more covers of particle filtration material (such as, unfilledmicroporous PTFE membrane); (3) covering the catalytic felt with one ormore covers of other filtration material (such as microporous PTFEfilled with one or more kinds of catalytic material; (4) covering thecatalytic felt with two or more covers of different constructions, suchas two different covers, each providing a different catalytic material;(5) constructing a filter apparatus with multiple layers of catalyticfelt (with similar or different construction); or (6) combining any ofthese or other filter constructions. The different layers may be mountedby lamination felting, needle felting, or without binding mechanism.

Without intending to limit the scope of the present invention, thefollowing examples illustrate how the present invention may be made andused:

EXAMPLE 1

A slurry of 2170 g TiO₂, V₂ O₅, WO₃ such as catalyst particles convertedfrom BASF monolith filter available from BASF Ludwigshafen, Germany(converted from BASF Catalyst O-85) was prepared in a 40 litercontainer. While the slurry was agitated at about 300 rpm, 1280 g PTFEin the form of a 27.6% solids dispersion was rapidly poured into themixing vessel. The PTFE dispersion was an aqueous dispersion obtainedfrom E. I. duPont de Nemours and Company, Wilmington, Del. About 30 sec.later, 3.4 liters of a 0.4% solution of modified cationic polyacrylimidesolution (such as SEDIPURE solution from BASF) in deionized water wasadded. After adding SEDIPURE solution, the mixture coagulated within 1min 45 sec. The coagulum was gently poured over a porous cheesecloth andallowed to air dry. The filtrate from this process was clear.

The coagulum was dried in a convection oven with a maximum temperatureof about 165° C. for 24 hours. The material dried in small, crackedcakes approximately 2 cm thick and was chilled to below 0° C. Thechilled cake was hand-ground using a tight, circular motion and minimaldownward force through a 0.635 cm mesh stainless steel screen, then 0.64g of mineral spirits per gram of powder was added. The mixture waschilled and tumbled for 5 minutes, then allowed to sit at 25° C.overnight and was retumbled for 5 minutes.

A pellet was formed in a cylinder by pulling a vacuum and pressing at853 psi. The pellet was then heated to 49° C. in a sealed tube. Thepellet was then extruded into a 6"×0.080" tape form. The tape was thencalendered through rolls to a thickness of 0.020 inch. The lubricant wasevaporated by running the tape across heated rolls. The tape wasstretched in the machine direction at a 2 to 1 ratio, 270° C., 105ft/min. A thin porous tape was produced with a porosity of approximately84% and a final thickness of about 10 mils.

EXAMPLE 2

A felt of the present invention was produced in the following manner. Acatalytic active ePTFE fiber was produced from a catalytic tape of ePTFEproduced from a wet mixture of 50% by volume titanium dioxide catalyst(completed from BASF O-85) and 50% PTFE resin (duPont) in accordancewith the steps outlined in Example 1, above. The tape was slit along itslength into three strips, expanded, and processed over a rotatingpinwheel to form a tow yarn. The tow yarn was then chopped to 5 cmlength staple fiber.

A similar staple was also produced from a synthetic fiber of expandedporous PTFE. The catalytically active ePTFE fiber and the syntheticfiber of expanded porous PTFE were opened in a shear air field and thencollected in boxes for additional processing. A blend of the two staplefibers was then produced by hand carding (mixing) 50% by weightcatalytic ePTFE fiber and 50% by weight synthetic fiber. The hand cardedfilter material was determined to be catalytically active and has aremoval efficiency for NO₂ of over 80%.

EXAMPLE 3

A woven scrim is made out of 440 decitex ePTFE weaving fiber, (RASTEX®fiber, available from W. L. Gore and Associates, Inc., Elkton, Md.). Thescrim is constructed with a thread count of 16×8 threads/cm resulting aweight of approximately 130 g/m².

A catalytic felt can also be made using the blend of the two staplefibers made according to Example 2.

The blended staple fiber from Example 2 is fed into lab-scale cardingequipment (a Davis & Furber carding machine available from Davis &Furber of Andover, Mass.). The carded web is crosslapped onto a scrimand tacked together by a needle loom. A web is then crosslapped onto theother side of the scrim and needled again. The felt is needle punchedseveral times to interlock the staple fibers to the scrim sufficiently.This product is then heat set while being restrained in the crossmachine direction for several minutes to improve the thermal stability.The final felt has a weight per unit area of about 1200 g/m², an airpermeability of about 0.75 m/min @ 12 mm water gauge, and a thickness ofabout 2 mm.

The needle felt is coated with a fluorinated ethylene propylenecopolymer (FEP) aqueous dispersion (T120 available from E. I. duPont deNemours and Co.). The felt is then dried in loop dryer oven at 200° C.with a dwell time of 8 min. The dried aqueous dispersion add on is 3.5%by weight.

A layer of porous expanded PTFE membrane with an air permeability of 8.8m/min @ 20 mm water gauge is laminated to the coated side of the felt.The felt is subject to sufficient heat, pressure, and dwell time tosoften the dried FEP aqueous dispersion. The resulting fabric laminatehad good strength between the porous expanded PTFE membrane and thefelt, and an air permeability of 2.8 m/min @ 12 mm water gauge withexcellent filtration efficiency of solid particulates.

The inventive nonwoven fabric laminate is tested to determine itscatalytic gas removal efficiencies.

The current invention can be used in many flue gas cleaning processesespecially to clean emissions of stationary sources. The filter materialcan be used to make filter devices such as filter bags or cartridges. Inmost cases these filters have to collect substantial amounts of dustthat is generated in the combustion process. After short collectiontimes between 1 and 60 minutes the collected dust layer on the dirtyside of the filter material increases the pressure drop across thefilter and the filter has to be cleaned. During this cleaning cycle thatinvolves a high energy input into the filter the outer dust layer fallsoff and a new particulate filtration cycle can begin. The catalyticconversion of pollutant gases will continue uninterrupted. Mostcatalytic filter materials that have been proposed to date consist of aregular woven or non-woven filter material in which the catalyticallyactive particles are inserted as a foreign body. These materials are notcommercially available. During the cleaning cycle in which the filtermaterial is exposed to high energy input and flexing these particles mayabrade the host fibers and degrade the life of the filter. In thecurrent invention the catalyst particles are an integral part of thefilter material and have no negative effects on the lifetime of thefilter material. Furthermore, all particle insertion devices have thedisadvantage that they increase the pressure drop of the filter materialand can get in contact with catalyst polluting dusts. In the currentinvention the catalyst particles do not interfere with the particlefiltration process at all and can not be deactivated by the pollutantdusts. Finally, the present invention allows for the ready combinationof filtration and catalytic functions, presently performed by separateapparatus, into a single easily employed unit.

The current catalyst works well at temperatures of 150° to 250° C. andpreferably between 200° and 250° C.

While particular embodiments of the present invention have beenillustrated and described herein, the present invention should not belimited to such illustrations and descriptions. It should be apparentthat changes and modifications may be incorporated and embodied as partof the present invention within the scope of the following claims.

The invention claimed is:
 1. A method of using a catalytic filtercomprising a coherent catalytic filter material and microporousmembrane, said method comprising:providing a catalytic filter comprisingmultiple fibers, each comprising catalytic particles attached to nodesand fibrils within a fibrillated polymer, the multiple fibers beingcombined to form a coherent filter material, and a sheet of microporousmembrane mounted on one side of said catalytic filter material; placingthe filter in a fluid stream with the microporous membrane oriented inan upstream position to protect the catalytic filter material from atleast one of particles and contaminants suspended in the fluid stream;and passing a fluid stream through the catalytic filter, whereby thefluid stream contacts catalytic particles attached to the node andfibril structure of the filter.
 2. The method of claim 1, furthercomprising providing a second microporous membrane in a downstreamposition of the catalytic filter.
 3. The method of claim 1, wherein themicroporous membrane comprises expanded polytetrafluoroethylene.
 4. Themethod of claim 1, wherein the microporous membrane further comprisescatalytic particles.
 5. The method of claim 4, wherein the microporousmembrane includes sorptive particles.